DISTINCT ELEMENTS OF THE XSNA PROMOTER ARE REQUIRED FOR MESODERMAL AND ECTODERMAL EXPRESSION

Xsna, the Xenopus homologue of Drosophila snail, is expressed in both mesoderm and ectoderm. Expression occurs in all mesoderm initially but is down regulated in a tissue-specific fashion at the end of gastrula tion in a way that reveals the subdivision of the mesoderm before its derivatives are overtly differentiated. Xsna is also expressed in the ectoderm of the prospective neural fold from stage 11, in a distinct b and of cells surrounding the prospective neural plate, which we design ate the neural plate border. The deep and superficial ectoderm compart ments labelled by Xsna represent the prospective neural crest and the prospective roof of the neural tube, respectively. Xsna expression per sists in neural crest cells during their subsequent migration. The rol e of the Xsna promoter in creating this pattern of expression has been investigated by injecting fertilised eggs with constructs containing the 5' upstream sequence of the gene fused to a reporter. An element o f 115 base pairs (-160 to -45 relative to the transcriptional start) i s sufficient to drive appropriate reporter gene expression. The promot er does not contain a TATA or CAAT box and does not have a high GC con tent, but RNA synthesis starts precisely at 33 bases upstream to the t ranslational start. The start sequence can be deleted so that transcri ption is initiated elsewhere without affecting the expression pattern. The distribution of Xsna promoter activity within the embryo, examine d using beta-galactosidase (beta-gal) fusions, is similar to that of t he endogenous mRNA seen by in situ hybridisation. The contribution of elements within the 5' sequence have been assessed by comparing the ex pression patterns of constructs that have deletions in this region. Se quences from -112 to 97 are required for mesodermal expression and seq uences from -96 to -44 are required for ectodermal expression. The beh aviour of the injected promoter constructs differ in one important res pect from the endogenous gene in that expression in an animal cap assa y is not inducible by mesoderm-inducing factors but is inducible by ce lls of the vegetal pole.